In order to produce wood-based materials with a long service life, it has been known to chemically modify the wood and in particular to acetylate the wood. Thereby materials with improved material properties, e.g. dimensional stability, hardness, durability, etc., are obtained. Wood mentioned throughout the description is selected from the group consisting of wood elements and solid wood.
Processes for the acetylation of wood elements, such as chips, embody particular challenges. These are generally different from the challenges encountered when acetylating solid wood, such as beams. Issues relate to the desired homogeneity of the acetylation, to the preference of providing a continuous acetylation process, and to the amount of the acetylation obtained. In both cases of wood elements and solid wood, obtaining of acetylated wood with a high acetyl content is a challenge.
A background reference on the acetylation of wood elements is WO2011/95824. Therein a two stage process is disclosed for the acetylation of wood elements in which the elements are treated with acetic anhydride (i.e. an impregnation step) and then further heated with an inert gas so as to complete acetylation (an acetylation step). As an optional first step, prior to the introduction of acetic anhydride, a vacuum may be applied to the elements to remove resident gases.
The impregnation with acetylation fluid of wood elements under vacuum is also addressed in WO 2013/139937. Therein a process is disclosed that judiciously combines a technique for batch-wise impregnation in such a way that the subsequent acetylation can be conducted in a continuous manner. Another background reference, also disclosing a two-step process, is U.S. Pat. No. 5,608,051.
It is notoriously difficult to obtain high acetylation degrees for wood. In WO 2013/139937 this is successfully addressed with a focus on the impregnation step. However, it would also be desired to further optimize the acetylation step. One desire in this respect is to improve the acetylation content. Another such desire is to provide a still better controlled acetylation process. Yet another such desire is to provide a novel alternative for existing acetylation processes.